Although it seems like an elementary question, the issue of colouring styrofoam is more problematic than one might think. Styrofoam, while an ideal craft and building medium, is difficult to colour. Spray paint it, and the paint and aerosol eat right through the styrofoam. Colour it with markers, and the slightest pressure leaves a ridge in the material in the marker's wake. Paint it, and the paint balls up and falls off or collects in the craters of the material, leading to splotchy results. Colouring styrofoam without destroying it in the process can be quite difficult. Fortunately, there is one way to colour styrofoam evenly and uneventfully.
Prepare the shoebox. Make a slit in the shoebox for every styrofoam piece that you will be colouring. The slits need to be far enough apart from each other that the pieces of styrofoam will not touch. If necessary, use more than one shoebox.
Squirt enough glue into the paper plate to cover the bottom. It does not need to be a thick layer, but you do need to cover the lower surface of the plate.
Mix food colouring into the glue. Use the paintbrush to mix it thoroughly. Start out by adding one or two drops, then add more to make the colour brighter. Stop when the colour is the right hue for you.
Roll the styrofoam pieces in the coloured glue. Use the paintbrush to get the colours in every crack and crevice.
Stab a plastic knife into the piece of styrofoam. This will serve to keep your styrofoam piece entirely out of contact with a surface that will mar your new paint job. Insert the plastic knife into a slot in the shoebox to keep the styrofoam piece suspended.
Let the styrofoam pieces dry for 24 hours. This should be plenty of time for the glue to set. Then, you can remove the knives from the styrofoam pieces.
Rinse out the paintbrush as soon as you are done with it and in between each colour to avoid damaging the brush or muddying your colours.